Chemical and biological control – hard and soft selection

Modern society has come to rely heavily on chemicals to control animal and plant pests, which are colonists that threaten people and their resources. The discovery of antibiotics led to cures of life-threatening infections and also eliminated the need for complex manual sterilizations of operating theaters and clinics. Although hospital costs were accordingly reduced, the microorganisms immediately adapted by the development of resistant strains from their survivors. Cavalli and Maccacaro (1952) described the consequence of such selection in the bacterium Escherichia coli using a regime where asexually reproducing colonies were plated on near-lethal levels of chloramphenical, so that selection can be regarded as being relatively hard. Those colonies surviving chloramphenicol did so because of a rare mutational event. In the absence of recombination, each new mutation was, therefore, tested on a particular genetic background that remained intact through subsequent (asexual) generations. Consequently in each selected line, the successful mutations may have conferred high resistance largely through their interactions with the rest of the genome. As long as asexual reproduction continued, these favorable epistatic interactions should persist. Accordingly, in sexual crosses, recombination disrupted these gene combinations that generated resistance, thus revealing that different lines had achieved resistance to chloramphenicol by different genetic routes.